Wide angle IR optical systems have a variety of applications, such as in surveillance systems, tracking systems and other wide angle imaging systems. FIG. 1 illustrates a prior art wide angle IR optical system 10. For example, the illustrated system 10 can have an optical speed of F/2 meaning that the ratio of the effective focal length (EFL) of the optical system 10 and the diameter (D) of the entrance pupil is 2. The optical system 10 can, for example, have a 20°×20° field of view (FOV). The optical system 10 includes a negative lens assembly 14 utilized to collect scenes from a wide FOV 12 and a positive lens assembly 20 for focusing the incoming beams onto an image detector 26. The negative lens assembly 14 is a first doublet that includes a negative lens 16 and a positive lens 18. The positive lens assembly 20 is a second doublet formed of a negative lens 22 and a positive lens 24. The positive lens assembly 20 includes an annular stop 21 on a front surface of the negative lens 22 for defining light beam bundles to be focused onto the image detector 26.
The performance of the system 10 is reasonable, if the application is for a single color (i.e., single wavelength bandwidth). However, two important issues arise, when multi-color performance is required. First, there is little room for inserting an optical channel for an additional color. Secondly, even if one or more beamsplitters and the associate optical systems for the additional colors can be inserted, the aberrations (astigmatism, coma and spherical) introduced by the beamsplitters are extremely difficult to compensate. To minimize these aberrations, a convention optical design would require a sophisticated set of focusing lens that includes a compensator, which would substantially complicate the system design and would have many disadvantages, such as increasing weight, package volume, fabrication and alignment costs.